"While the downing of the aircraft may constitute a war crime, a thorough, effective, independent and impartial investigation is needed to determine the facts and circumstances of this act," Ivan Simonovic, UN assistant secretary-general for human rights, said while briefing the UN Security Council on the current situation in eastern Ukraine.

"This investigation is now underway, led by the Netherlands," Xinhua quoted him saying.

Simonovic said that it was disturbing to learn that the volatile security situation at the crash site continues to hamper the investigators, despite the Ukraine government declared ceasefire zone around the area.

"It is urgent to stop the fighting and to secure the crash site, " Simonovic said.

Simonovic said there needs to be accountability for those responsible for war crimes, serious violations of international humanitarian law and gross violations of human rights law, as documented by the findings of the UN Monitoring Mission.

The Boeing 777 went down July 17 in Ukraine near the Russian border, killing all 298 people on board.

Four Dutch and Australian experts last week reached the crash site in eastern Ukraine, said the monitoring mission of the Organization for Security and Cooperation in Europe (OSCE).

Fighting between the Ukrainian government forces and pro-independence insurgents in the region had prevented international investigators from reaching the site.

The Ukrainian authorities said Monday that the international investigation commission pointed to "massive explosive decompression" as a possible cause of the disaster after an examination of the content of the black boxes.